Quantifying sediment disturbance by bottom currents and its effect on benthic communities in a deep-sea western boundary zone

Deep-SeaResearch,Vol. 36, No. 6, pp. 901-934, 1989.

0198-0149/89 $3.00 + 0.00 ~ 1989PergamonPressplc.

Printedin Great Britain.

Quantifying sediment disturbance by bottom currents and its effect on benthic communities in a deep-sea western boundary zone JOSEPHINE Y . ALLER*

(Received 30 June 1988; in revised form 23 December 1988; accepted 4 January 1989) Abstract--Erosion, transport and redeposition of sediment by near-bottom currents are major sources of disturbance for soft-sediment habitats and associated benthic communities. This phenomenon takes place in western boundary slope regions of the deep sea such as the HEBBLE area on the Nova Scotian Rise, western North Atlantic. Bottom disturbance in this western boundary region can be characterized and quantified, first in terms of the driving force--the current and directly related bed shear stress; and second, by the expression of the current effect as observed in sedimentary fabric, %CaCO3, and granulometry. These physical characteristics can be correlated with biologic features, including abundances and activities of sediment microorganisms, and apparently, in abundances and distributions of meio- and macrofauna. Currents measured at heights of 1-59 m above the seabed at the HEBBLE site (4815-4830 m) from February 1982 to 15 September 1986 show evidence of "benthic storms" with current speeds of 15-23 cm s-1 for >~2 days. These "storms" occur with a frequency of about 21 days and have mean durations of 7 + 5.8 days. Storms with mean velocities over 23 cm s-1 occur every 10 months and last 12 _+ 11 days. X-radiographs of vertical slabs of sediment taken from box cores at the HEBBLE site show stratification features related to current speeds and bed shear stress, immediately preceeding the time of core collection. These relationships are corroborated by radiochemical distributions of 23"I'h. Both erosional and depositional processes affect physical and chemical properties of the sediment and have positive and negative effects on the benthic community. Erosional periods result in sediment transport and sweeping of surficial organic matter, micro-organisms, larvae and juveniles from the area. During transitional periods of intermediate current velocities there is deposition of fresh organic matter, removal of metabolites, and mechanical stimulation of sediment micro-organisms. Periods of decelerating current speeds result in rapid deposition of several cm of sediment on to the seabed, burying organisms and filling-in burrows. Benthic macro- and meiofaunal abundances are maximum during these depositional periods. Periods of low current speed are not necessarily periods of low physical disturbance.

INTRODUCTION

THE erosion, transport and redeposition of sediments are major sources of physical disturbance for soft-sediment habitats and associated benthic communities. These sedimentological processes are commonly encountered in nearshore and shelf habitats. In this study I will show that bottom disturbance by sediment transport is also characteristic of western boundary zones. Unlike many shallow water cases (e.g. WHITLATCH,1977; RHOADS et al., 1978; TENORE et al., 1978; SANTOSand SIMON, 1980; RHOADSand BOYER, 1982; ZAJAC and WHITLATCH, 1982; Yn~GST and RHOADS, 1985; ALLER and ALLER, 1986b), disturbance in western boundary zones occurs in a benthic environment characterized by low rates of input and low inventories of labile organic matter. * Marine Sciences Research Center, SUNY at Stony Brook, Stony Brook, NY 11794-5000, U.S.A. 901

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Because of the differences in the lability of the detrital pool between shallow water and deep water disturbance regimes, this comparison has potentially unique and theoretically interesting implications. One of the difficulties in examining the impact of the currentderived disturbance on the benthic fauna has been the inability to relate quantitatively changes in the flow regime to properties of the sediments and characteristics of the bottom fauna. In fact, methods for measuring disturbance have been largely neglected in the general literature (PEET et al., 1983; ARMESTOand PICKETr, 1985; PICKETr and WHITE, 1985). In this paper, a method is given for characterizing and quantifying bottom disturbance in a western boundary zone. Criteria for inferring sedimento!ogical effects of these current-derived disturbances on the bottom are developed from sedimentologic properties preserved within the upper layers of the deposit. Finally, the relationships between disturbance and the sediment microbial, meio- and macrofaunal communities are examined. As far as I am aware, no such operationally consistent quantitative description of sediment disturbance has been presented previously, at least not within a marine ecological context. STUDY AREA

The study site is on the lower continental rise off Nova Scotia (48°27'N, 62°20'W) (Fig. 1) in water depths of between 4815 and 4830 m. This study area (Fig. 2), lies beneath two major currents: the Western Boundary Undercurrent (WBUC) (<4000 m) and the Deepwater Boundary Current (DWBC) (>4000 m), where daily averaged current velocities of 10-25 cm s-~ (10 m above the seabed) can occur for periods of several days (RICHARDSONet al., 1981; WEATHERLYand KELLEY,1983). These velocities are significantly higher than the - 3 c m s-1 cited as typical of abyssal plain conditions (MuNK et al., 1970) and are among the highest ever measured in the deep sea (RICHARDSONet al., 1981). Particles entrained by these currents produce near-bottom suspended sediment concentrations greater than those of nepheloid layers measured in many other locations throughout the world's oceans (Biscaye et al., 1980). Periods of strong near-bottom currents (i.e. "benthic storms" sensu WEATHERLYand KELLEY,1985) erode and transport sediments. During intervening periods of weaker flow, sediment deposition occurs (YINGSTand ALLER,1982; ALLER and ALLER, 1986a). The region has a slope of 0.17-0.57 ° (GRANTet al., 1985). The sediments are poorly sorted and are comprised largely of silt and clay-sized particles mixed with larger particle aggregates, including fecal pellets. Arenaceous and calcareous foraminiferan tests comprise a significant part of the sand fraction, and reworked lag deposits of gravel are present (McCAVE, 1985). The sediments are more consolidated (i.e. lower water content) than those of similar textural and qualitative composition from shallow water (YIN~ST and ALLER,1982) and are usually reddish brown (i.e. oxidized) to at least 10 cm. Pore water NO~ profiles indicate that net nitrification is taking place over the upper - 3 - 5 cm and denitrification processes exist to a depth of at least - 2 0 cm (R. ALLER, personal communication). Organic carbon concentrations (0.4--0.9%) and solid phase nitrogen (0.05-0.13%) in the upper 5 cm of the general sediment are typical of other western boundary zone sites such as the Venezuela Basin (0.33-0.078% organic C; HARVEYet al., 1984) and Demerera Abyssal Plains (0.6-0.8% organic C; 0.~7-0.14% N; Rowe and DEMING, 1985). Organic carbon and nitrogen concentrations in localized areas

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associated with burrows, however, can be twice as great as ambient values (ALLER and ALLER, 1986a). SAMPLING

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METHODS

Sampling dates, cruise information, and the relative current velocities (from Fig. 3) during the sampling periods are presented in Table 1. Station locations during each of the five cruises to the site are shown in Figs 1 and 2. All moorings for collection of current records (Fig. 3), suspended particle concentrations and bed shear stress were located at the H E B B L E site within 1-2 km of the location of all box cores (Table 2). These data are used in the present paper to describe hydrodynamic conditions near the seabed.

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Table 1. Information on sample collection at H E B B L E site, Nova Scotian Rise. Location o f individual box cores at each station are indicated in Fig. lB. K96 (July 1982), K101 (April 1983), K103 (June 1983), K106 (August 1984), K126 (September 1986) Cruises/box core no. Relative current conditions and speeds (cm s-1) during cruises

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All bottom samples were taken from the R.V. K n o r r using a 0.25 m 2 box core (HESSLER and JUMARS, 1974) equipped with modifications originated by R. L. Hessler, P.A. Jumars and J. Finger to reduce the bow wave effect [see THISTLE (1983) for discussion]. Great effort also was made to minimize washing of the sediment surface during retrieval and processing of the box cores. The 0.25 m E box cores contained removable 5 x 5 cm subsamplers within a central set of nine 10 x 10 cm subcorers [see THISTLE et al. (1985) for description]. Narrow rectangular 2.5 cm (i.d.) thick acrylic subcorers were inserted into the box cores, and the vertical section of sediment was thus obtained, refrigerated and X-rayed aboard ship as quickly as possible, generally within several hours of collection. These X-radiographs were used to examine and quantify stratigraphic features within the sediment in order to relate these features to measured current speeds (or bed shear stress) and bioturbational activity. After X-raying, the sediment was extruded and sliced incrementally at the following depth intervals for grain size analyses: 0-0.25, 0.25-0.5, 0.5-1, 1-2, 2-3, 3-5, 5-7, 710 cm. The sediment was preserved in 0.3% gluteraldehyde in 3% NaC1 solution and stored in a refrigerator. Particle size analysis of the sand fraction (I>63 ktm) was done by gently washing the sample through a series of nylon mesh sieves using 3% NaCl. In this way, particle aggregates (including arenaceous foraminiferan tests) were not destroyed. Each fraction was subsequently rinsed with distilled water, dried and weighed. The percent sediment in each fraction was determined after converting the total wet weight of the sample to dry weight using the water content of that particular depth interval. The particle size distribution of the ~<63 ~tm fraction was determined by pipette analysis (FOLK, 1974) after addition of sodium hexametaphosphate (calgon) as a dispersant. For biological sampling, the sediment from each of the 5 × 5 cm subcores was extruded upward, sliced at the following depth intervals: 0-0.5, 0.5-1, 1-2, 2-3, 3-5, 5-7 and 7-10 cm, and subsequently divided for the following analyses: bacterial standing stocks, meio- and macrofauna, ATP concentrations, sediment water content, %CaCO3 and organic C. Gluteraldehyde-preserved 1 cc subsamples (0.3% gluteraldehyde in a 3% NaCl solution) were used to directly count bacteria by the epifluorescence method of HOBBIEet al. (1977) as modified by WATSONet al. (1977). Counting precision was determined from five independent counts of bacteria from one sample which covered a 42.5 cm 2 area and gave 95% confidence limits of 17.65 _+ 0.69 × 109 cells g-1 dry sediment. Sediment adenosine triphosphate (ATP) concentrations were extracted onboard ship within 4 h with a boiling phosphate-citrate buffer (BULLEID, 1978). All samples were extracted in duplicate and frozen. Each sample extract was later thawed and ATP assayed using the firefly bioluminescent procedure. ATP concentrations were calculated using a series of standard ATP recovery curves determined with ATP-free sediment from this same area. These curves allow correction for extraction efficiency and chemical interference as a function of weight of sediment extracted (ALLER, unpublished data). Densities and ATP concentrations are given per g dry weight of sediment. Bacterial abundances and ATP concentrations are based on nine separate samples. To verify within and between cruise variability, all data were normalized to the fine-grained fraction (%~<63 ~tm) due to spatial variability in the % silt/clay fraction. Sediment water content was estimated by weight loss of initially wet sediment samples dried at 60°C. Organic carbon was determined by wet oxidation (GAUDETrE et al., 1974: precision --+1%).

Quantifyingsedimentdisturbanceby bottomcurrents

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Direct inorganic carbonate analyses were made using a gasometric procedure described by SCrtINI
DISTURBANCE--METHODS

AND RESULTS

Two components are involved in the characterization of disturbance in the study area. First, the driving force--the current defined here by magnitude or speed (cm s-l), by duration (time) and by frequency of occurrence (events/unit time) and second, the expression of the current effect in the sedimentary fabric. Currents can be directly related to bed shear stress which, in turn, may result in sediment resuspension. In quantifying the driving force, I will focus on those currents that result in the highest shear-forces. WEAa~aERLY and KELLEY(1985) have labeled these as "benthic storms": periods when current velocities are maintained at I>15 cm s-t for two or more consecutive days. The frequency of disturbance or its inverse, the reoccurrence interval (measured as the time per event), indicates how often the seabed, an organism or the entire biological community, is exposed to a sediment transport event.

Relationship of measured current velocities with measured bed shear stress Omnitripod data of WILLIAMSand GROSS (unpublished data) from a 60-day period in 1986 (16 July-13 September) relate measured current speeds to bed shear stress at the HEBBLE site (40°26.84'N, 62°21.85'W). Over the same time period, calculated bed shear velocities (based on direct measurements of kinetic energy and Reynolds stress estimates at 82 cm above bottom) are shown in Fig. 4B. The shear velocity was found to correlate extremely well with current velocity (r = 0.97) (Fig. 4C).

Characterization of benthic storms A partial 4 year current meter record at the HEBBLE site (4815-4825 m) is shown in Fig. 3. This figure shows storm frequencies, storm durations and storms magnitudes for at least 30 days prior to five cruises made to the site for collection of biological and sedimentologic samples. Current records during this period were made at heights of 159 m above bottom. No velocity profile corrections were necessary, as all velocities were within +10% and -5% of the "free stream" value as measured at 200 m above the bottom during a 6-day experiment (WEATHERLYand KELLEY,1985). Data on benthic storms at the HEBBLE site (Table 3) show 70% predominance of westward-flowing storms, with intermediate periods (30%) of eastward flow reversal (Fig. 5). The relationship between the frequency of storms of a given magnitude and the frequency of storms of the same duration can be seen in the three-dimensional plot in Fig. 6. Storms with peak velocities between 15 and 23 cm s-1, for example, occur on the

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order of every 21 days and last 7 ___ 5.8 days. Storms of over 23 cm s-1 occur relatively infrequently (once every 10 months), but tend to last 12.3 + 11.4 days. Storms of greatest magnitude (31-33 cm s-1) are rare, occurring once every 2 ½to 3 years. The expression of the currents in the stratigraphic record The second component of the characterization, the expression of the current effect in the sedimentary fabric, is demonstrated in representative X-radiographs (Figs 7-9) which illustrate a variety of structures which result from (1) the physical reworking by currents (i.e. buried erosional contacts, cross-bedding or cross-lamination, and pebble-lag deposits), and (2) biological activities including mixing and burrowing by infaunal orga-

Quantifying sediment disturbance by bottom currents

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Table 3. Information on benthic storms at HEBBLE site. Data compiled from W~THERLY and KEt.LEr (1985), GROSSand WILt.IAUS (unpublished data) and WEArnERLY (unpublished data) Benthic storm 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Date 02/27-03/01/82 03/09-03/18/82 03/24-03/27/82 04/03-04/06/82 04/25-05/14/82 06/18-06/29/82 10/12-10/15/82 11/03-11/10/82 11/17-11/23/82 ff2/09--02/17/83 03/08--03/24/83 04/28--05/01/83 06/07--06/09/83 07/26--07/29/83 09/22-09/24/83 10/22-10/24/83 10/27-10/30/83 01108-01111184 02/17-03/14/84 03/26--04/07/84 04128--05109184 08/01-08/24/84 04/14-04/27/86 05/11-05/14/86 06/14--06/17/86 06/28-07/05/86 07/22-07/26/86 08/01-08/04/86 08/07-08/25/86

Maximum daily avg. current (cm s-1) (°T)* 21.3 22.1 28.1 19.1 26.7 22.7 21.4 28.5 22.0 19.5 31.4 16.0 15.0 19.5 17.3 18.0 20.5 17.8 18.4 21.3 28.9 23.1 20.4 16.9 19.2 17.7 19.7 21.9 22.7

94 67 283 230 257 79 266 261 57 262 263 253 246 259 225 230 180 220 196 240 227 260 243.5 228.5 246.1 243 63.1 120.5 212.3

Duration (days) 4 11 4 4 20 12 4 8 9 9 17 4 3 4 3 3 3 4 24 14 12 24 14 4 4 8 5 4 19

Suspended particle concentration (Ixg-t)t 421 447 661 354 589 465 425 644 444 366 734 258 228 366 193 + 125 269 + 81 280 + 86 1074 + 978 363 + 242 681 + 497 472 + 241 261 + 978

460 + 216 248 + 223 660 + 521

* °T refers to current direction where N = 00, S = 180°, E = 90° and W = 270°. t Suspended particle concentrations which can be linearly related to attenuation coefficients obtained from transmissometer readings (GARDNERel al., 1985). No transmissometer data was available for storms 23-26. Daily averaged suspended particle concentrations ±S.D. are given where available.

nisms. L a m i n a t i o n s , s i m p l y q u a n t i f i e d b y m e a s u r i n g t h e v e r t i c a l t h i c k n e s s o f surficial l a m i n a t e d i n t e r v a l s , o c c u r o n two scales: fine l a m i n a t i o n s ( a b o u t 1-2 m m in t h i c k n e s s ) , visible p a r t i c u l a r l y n e a r t h e s e d i m e n t surface, a n d t h i c k l a m i n a t i o n s ( > 1 c m ) , w h i c h o c c u r t h r o u g h o u t t h e d e p o s i t . T h e s e i n d i v i d u a l l a y e r s ( b a s a l d i s t a n c e Zi f r o m t h e s e d i m e n t - w a t e r i n t e r f a c e ) o f v a r y i n g t h i c k n e s s a r e s e e n in an X - r a d i o g r a p h o f s e d i m e n t at Sta. K 9 6 - X V I (Fig. 7B). T h e d i s t a n c e b e t w e e n t h e b a s e o f a n y t w o a d j a c e n t l a y e r s ( Z = Z i + 1 - Z i ) is a m e a s u r e o f t h e t h i c k n e s s o f a specific l a y e r . T h e s u m m e d t h i c k n e s s o f i n d i v i d u a l l a y e r s (Y.i(Zi+l - Zi) o r Zi ( m a x ) ) s e e n in a p a r t i c u l a r r a d i o g r a p h gives t h e t o t a l d e p t h o f t h e p h y s i c a l l y r e w o r k e d z o n e ( Z ) in t h e d e p o s i t (Fig. 7C). T h e s t r a t i g r a p h i c t h i c k n e s s e s o f c r o s s - l a m i n a t i o n s in t h e n e a r - i n t e r f a c e r e g i o n ( < 1 0 15 cm) w e r e m e a s u r e d in ~ 7 0 X - r a d i o g r a p h s (at least t w o p e r b o x c o r e ) t a k e n o n t h e five cruises to t h e H E B B L E site. F o u r to six m e a s u r e m e n t s w e r e a v e r a g e d a c r o s s t h e r a d i o g r a p h for t h e d i s t a n c e b e l o w t h e i n t e r f a c e o f a n y g i v e n l a m i n a t i o n (Zi). T h e Zi a r e p l o t t e d a n d c o m p a r e d with d a i l y a v e r a g e d c u r r e n t m a g n i t u d e s r e c o r d e d o n t h e day t h e r a d i o g r a p h was c o l l e c t e d (Fig. 10). T h e s e d a i l y a v e r a g e d v a l u e s w e r e u s e d for o p e r a -

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tional convenience but can be shown to be correlated extremely well with longer term averages (Fig. llA,B). The relationship between the daily averaged current velocity for each day during a cruise period and the averaged velocity for the preceeding 7, 14, 21, and 28 days were established by computing product-moment correlation coefficients (SOKAL and ROHLF, 1969). The association of core collection day averages with the previous 7-day averaged velocities (R = 0.97) is illustrated in Fig. l l A . Figure llB shows that the daily velocities are positively correlated with 21-day averaged velocities (R = 0.75). During periods of low current velocities, a maximum number of cross-laminations and depth of layering occur in randomly collected cores (Fig. 10). These are interpreted as depositional features, an interpretation supported by CaCO3 distributions and radio-

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sta.I

.

.

.

.

.

.

.

.

.

.

Fig. 8. Vertical profile of %CaCO3 (A) and representative X-radiograph (B) from Sta. K103-I. Pebbles (p) and relatively large burrows are apparent throughout the top 10 cm in this Xradiograph. Bar = 1 cm. The absence of (1) a porous surface layer, (2) extensive burrow networks near the interface indicating recent biogenic reworking, (3) visible erosional contacts, and (4) a relatively constant CaCO3 profile are correlated with periods of intermediate current flow and recent net erosion. C a C O 3 values in the top 1 cm are the mean ±S.D. of three separate samples.

913

Quantifying sediment disturbance by bottom currents

A

caco

.

4,

3 (~)

', i 2, ] ~ 20,

B. lcm

~ 2,B,

!

I

+

3

_-L

O I

5

n W 0

7

g KiO6

-,'ca. T

Fig. 9. Vertical profile of %CaCO 3 (A) and X-radiograph (B) at Sta. K106-I showing a very thin (<1-2 mm) surface layer and one erosional contact (1) at 3 cm. Bar = 1 cm. Burrowing by irregular echinoids (E.b.) produce locally homogenized areas which appear round in crosssection. A spiral burrow is also evident. A prominent erosional contact at 3 cm correlates with a two-fold increase in CaCO 3 (marked by dashed line).

Quantifyingsediment disturbance by bottom currents BENTHIC

STORMS

_

AT

HEBBLE

915 SITE

(6/82-9/86)

3.56I

rr w 2-.68

0 Z )0 Z Ill D 0 111 fT 11.

t .80

,4

: fVV":

I MAGNITUDE

(CM

S E C -±)

Fig. 6. Three-dimensional plot relatingthe frequency of storm occurrence in the H E B B L E

area to the mean current magnitude of individual storms and the duration of such storms. A storm is definedas periods when current velocitiesare maintainedat ~>15cm s-~ for two or more consecutive days (sensuWEATrIERLYand KELLEY,1985). chemical distributions to be shown later. As current velocities increase, particle resuspension or bedload transport increases and the number and prominence of the preserved laminations decreases. During cruise K106, sampled during a period of strong currents (x = > 25 cm s-~), both the number of surficial layers and their total thickness were at a minimum. Again, an interpretation of net erosion during these times is independently supported by radiochemical data to be provided later.

Temporal patterns in sedimentologic properties The spatial distribution of the >63 lam particle fraction (weight percent) averaged over the upper 2 cm at all stations within the H E B B L E site is shown in Fig. 12. Because of spatial variability in the sand fraction in surficial sediments (Table 4), temporal changes are most noticeable when vertical profiles from each cruise are compared for individual stations (Fig. 13). During periods of relatively low currents and net deposition [July 1982 (K96) and September 1986 (K126)], in the top 0.5-1 cm at all stations, the silt and clay fraction is higher, and the percent sand fraction substantially lower compared to periods of increased current velocities and net erosion [April 1983 (K101), June 1983 (K103) and August 1984 (K106)]. At Sta. I, for example, the percent sand in the top 0.25 cm doubled from 12.4% in July 1982 to 25% in June 1983. Diluted by fine particles, the percent sand decreased again to 5.8% in September 1986 (Fig_ 13).

916

J.Y. ALLER

CURRENT

SPEED

(CM 3

i

SEC--I)

9

•

i

~13, ~

un2n7

2 1

K101

K103

•

m

•

KIO@

-~

•

•

÷

E W []

[] D

A

[] []

.+ ¢, K 9 6 ~

Z 0

•

~

5

H

H

OSZON 0 A

126( ~

+

Z

A

H

E 7 /

-÷ 0 • •00 q~ 0 0 ~

0 •

0 9 DEPOSITION

<>

A

Fig. 10. Quantitative expression of the current effect in sediments from the HEBBLE site. Vertical thickness of large-scale physically reworked layers (Zi) measured in the near-interface region in - 7 0 X-radiographs is compared with daily averaged current magnitudes measured on the day the radiograph was collected. Daily averaged velocities correlate well with averaged velocities for the preceeding 1-4 weeks (Fig. 11). Depths of individual layers are measured as the distance below the interface and are displayed with the following symbols: solid = first layer (Z1); open = the second layer (Z2); and slashed = the third layer (Z3) as diagrammed in Fig. 7C. Cruises during which radiographs were taken and current magnitudes recorded are: K96 (diamonds), K101 (circles), K103 (triangles), K106 (squares) and K126 (* (Z1) and + (Z2)). The curved line approximates the maximum thickness of the physically reworked zone (distance below interface to deepest discernable contact Z ~ ) as a function of the mean current magnitude averaged over a 7-day period prior to core collection. A very similar figure could be drawn relating maximum physically reworked depth to a 21-day average. The interpreted transition from periods of relative deposition when current magnitudes are low to periods of relative erosion with high current magnitudes is shown.

917

Quantifying sediment disturbance by bottom currents

27

0 W W n

<:~

K~.O6

<3 <3

I >[]

(~ ±5

K~Oa

X

h

x

9

~

W

~

¢-o-.

xx

Z

X

X

X

~

~

o

~

o

K~26

K95 (R12-.97)

3

t

I

9

I

I

I

15

DAILY

MEAN

(CM

I

I

I 21

I

27

SPEED

S E C - - 1)

(B) 27 o w IH

~

21

>o

~e

fu Z

X°~oX

× ~ ....

9

w I[

~

*

dlm

3

(R12

3

I

I

I

9 MEAN

I

I

I

15 21 DAILY SPEED (CM S E C - - 1)

-

I

27

.75)

I

Fig. 11. Daily averaged current velocity for each day d u r i n g a cruise plotted against the averaged velocity for the preceeding 7 ( A ) a n d 21 ( B ) days. Excellent correlation demonstrates that short-term conditions (e.g. current at time of core collection) reflect longer time scale processes (current over previous month).

Organic carbon in the top 0.5 cm decreased significantly at all stations (P < 0.05) between K96 (July 1982) and K101, K103 and K106, associated with periods of increased current velocities, net erosion and increased sand content (Fig. 14). Decreases in the organic C content occurred to at least 1 cm below the sediment surface at all stations, with consistent decreases throughout the top 10 cm at some stations (i.e. Sta. VI in Fig. 14) during times of increased current velocities and net erosion. The %CaC03 largely reflects the presence of calcareous foraminifera which comprise the sand fraction. Foraminifera increase generally with core depth, from 10 to 20% at the surface to >30% at 7-10 cm. Changes of +5% in CaCO 3 track major laminations or

918

J . Y . ALLER

•

oi.35

\

~x

~0

/

\

:I.e. 3 3 (2.3)

ii

Iti (

~71

~.~a 7 ~45 ( 5 , 48) " (3~77) 4.C8)

\

xv

'

o

%.

~o

-%

c

~ 44 18

#0

•

~7

2 . 74

)

•

. 7 E

,4830m

6 , 87.

-

6.

~5

(9.

34

S.O .°54

/

ramge~l

/

. 33--26. (n~30)

5,%

/ Xlll

IV

~lll o

~.3 ,e76

/

/

(:£6. 6

4820m

• \2,

%

5. 58

~.~

:~.,

(±2.5)

(3.56)

21~. 5 2

) 5

-%

\

/ Fig. 12. Weight percent distribution of ~>63 Inn particles averaged over the top 0-2 cm at HEBBLE site stations during K96 (July 1982) illustrating small-scale (within station = 0.5 x 1 km) and larger scale (over the site = 2 x 4 km) spatial variability. Figure from MCCAVE (1985) with my independent measurements in parentheses.

erosional contacts (Figs 7-9). During K96 (Fig. 7) more major laminations and more breaks in CaCO3(%) can be seen; while during K103 and K106, CaCO3 discontinuities may appear at a relic erosional contact (Fig. 8) but CaCO3 distributions are generally more uniform with depth (Fig. 9).

Biotic properties As shown above, the percentage of fine (<63 I~m) particles and organic carbon concentrations decreased on cruises K101 and K103. However, bacterial abundances and ATP concentrations increased dramatically during cruises K101 and K103 apparent in a representative temporal series of vertical profiles of bacterial standing stocks at Sta. XIII

919

Quantifying sediment disturbance by bottom currents

Table 4. Date Cruise 0-2 cm Station I

Temporal variability in percent sand (~>63 lam) distributions in the 0-0.25 and 0-2 cm depth intervals at four stations at the H E B B L E site. Data of McCAVE (1985) in parentheses July 1982

April 1983

June 1983

Sept. 1984

Sept. 1986

K96

K101

K103

K106

K126

16.59

12.38

ll.9

6.87

4.50

12.45 (9.00) 9.34 (6.95) 2.30 (1.33)

21.86

14.19

4.49

11.98

14.21

12.5 21.1 2.7 6.2

12.7 45.2 12.9 -

(21.05) XIII V VI 0--0.25 cm Station I XIII V VI

14.83

14.50

15.87

20.9 12.4 14.8 14.9

14.3 8.6 13.4

5.7 14.8 -

(Fig. 15). These two cruises corresponded to transitional periods of intermediate current velocities (bottom shear stress) and intermediate levels of surficial erosional disturbance. The most noticeable effect can be seen at all stations in the top two depth intervals especially when normalized to the ~<63 Ixm fraction (Fig. 16). Averaging all stations, bacterial standing stocks in the top 0.5 cm, for example, increased significantly (P < 0.05) between July 1982 (K96) from 2.3 + 2.2 x 109 g-1 (n = 7 stations) to 7.3 + 1.9 x 109 g-1 in April 1983 (K101) (n = 4 stations), remained elevated at 4.4 + 0.9 x 109 g-1 in June 1983 (K103) (n = 5 stations), dropped to 1.6 + 0.6 x 109 g-1 in August 1984 (n = 3 stations) and increased again to 5.6 + 2.0 x 109 g-1 (n m_ 3 stations). This same pattern of change over time at all stations is apparent, but less dramatic at depths of 0.5 - 1.0 cm and generally throughout the top 0-5 cm. As illustrated in a representative vertical profile (Sta. XIII), total sediment ATP concentrations were higher throughout the deposit, but particularly in the top 1 cm, at times of increased currents (Fig. 15). At Sta. IX, concentrations were as much as 6.5 times as high in K103 as K96 over the top 1 cm. These increases track significantly greater (P < 0.05) bacterial biomass and, apparently, activity rather than increased meiofaunal abundances, as discussed later. The temporal pattern of sediment ATP concentrations at all stations in the top two depth intervals (Fig. 17) clearly shows the significant (P < 0.05) (3 - 6.5 fold) difference between April (K101) and June (K103) 1983, periods of intermediate current velocities when concentrations were highest, and either periods of low current speeds (cruises K101 and K1036), or times of high currents (K106). Meio- and macrofaunal abundances (>41 I~m) at Sta. V, shown here as an example, decreased significantly (P < 0.05) in the top 0.5 cm from 708 + 262 (n = 9) individuals 23 cm-2 in July 1982, to 232 + 128 individuals 23 cm-2 (n = 9) in April 1983 (Table 5). Abundances of nematodes, foraminifera and polychaetes decreased the most. Densities in the 0.5-1 cm interval did not change between sampling times and remained at 132 individuals 23 cm-2 (P > 0.05).

920

J . Y . ALL, It

Particles

5

0.0 0.5 ~..0 1.5 EE o

5 Q_ (I)

2.0 2.5 3.0 3,5 4.0 4.5 5.0

10

15

¢

+

20

>0.063 25

¢+K9~

I t

2.5

T

"°'/

I

1 Station I

5

¢

10

>0.063 15

mm 20

(%) 25

',1,

+KI03

1.0

c-

25

Intermertiat~

'°F"°w+

°° I

4--J

20

::F.,oL "

().5

2.0

15

I

~FI'

0

u

t0

I

Particles

1.5

5

1"50[

Low Flow

(C)

(%)

~i0[ ++ +

+

K126

mm

(B) 0

K96

S t a t i o n VI

Fig, 13. Temporal changes in vertical profiles of weight percent particles ~>63 lun at Sta. I (A and B) and VI (C). Slow flow conditions during K96 and K126 when sand concentrations, particularly in the top 1 cm, were relatively low are contrasted with increased % sand during intermediate (K101 and K103) and high current flow and net erosion (K106).

Quantifying sediment disturbanceby bottomcurrents

921

(A) 0.9

0 - 0 . 5 cm

0.8 c o ,,7 £_ (13

0.7

0.6

~

(.J

O. 5

K96

[3

.,'4 C" Cn C (D

0

0.4

0

K10t K103

0.3

K106 I

i

O. 2

July

April June 1983

t982

i

Sept. 1984

TIME

Organic Carbon (%) (B)

0

• I0

.30

.50

.70

I

.9 =

0

'

'

Kgo"

2

Ki03

~[~

3 E (2

4

40 o. c~

6

K10.6,

5 ,

0

7 8

OI 9 I0

Station

VI

Fig. 14. (A) Temporal change in percent organic carbon in the top 0.5 cm at five stations, illustrating the effective removal of organic matter associated with fine particles during periods of increased current velocities. (B) Representative vertical profile at Sta. K96-VI illustrating a systematic decrease in organic carbon throughout the top 10 cm between periods of low current velocities and periods of intermediate and strong flow conditions.

DISCUSSION

Layering of sediment The alternating sandy-silty-clay (>90% <63 txm) layers distinguishable in the upper 10-15 cm of X-radiographs are produced by erosion-deposition cycles. These layers may be identical in bulk composition to each other, but the fact that they are recognizably distinct and separated by discontinuities in particle composition (i.e. concentrations of sand-size particles composed of calcareous foraminiferan tests) indicates separate depositional events. The silty-clay layers in the upper zone are distinct from the

922

J.Y. ALLER

(A)

Bacterla (~0 g g-l)

L!!++,

00

2

4

6

8

~(B) ~

2

~0

4

6

B

--,~

i

®

,[

K126

2

Bacteria (I09 g-l) 10

~-_ K101

2 IKf.03

3 4 5 6 7

:t

8 9 10 Station

XIII

ATP (ng g-~)

icl 0

5

o

,

10 ,

15

20

25

30

35

,

,0

5

10

15

20

25

30

o

,

,

,

,

,

,

II"

Intermed 1ate

tH!ii

2

u

ATP (ng g-~)

(O)

[

w Flow

3

35

K

Flow

~

4

r-..1 5

6

t

7

7

--

15. Representative temporal series of vertical profiles of bacteria (A and B) and total sediment ATP concentrations (C and D) at S t a . K 9 6 - X I I I illustratipg an overall pattern o f increase during periods of intermediate current velocities and net erosion ( K 1 0 1 , K 1 0 3 ) as compared with depositional periods and slow currents (K96) and periods of very strong currents

Fig.

(K106).

underlying turbidite layers that MCCAvE (1985) identified in box cores collected at Stas II, XIV and X (Fig. 2). A layer of terrigenous sand (>50%) appears at - 2 0 cm, and a layer of coarse sand and gravel appears at - 4 0 cm. Biogenic reworking is obvious above the turbidite at --20 cm, while the turbidite itself is laminated, probably as a result of physical reworking by currents subsequent to initial deposition (McCAVE, 1985).

Particle reworking and the preservation of physical structures In order to preserve complete sets of cross-laminations or bedding contacts within the HEBBLE site deposits, these contacts must be buried below the mean depth of biogenic

Quantifying sediment disturbance by bottom currents

923

(A) 1,4-

. 5CM

0-o

iO K~oz

t

6"

I

_ ....

K S03

~

2I

01 o

T---

I

I

I

I

I

K~Om I

I

I

(B) 14

0

.

5--I

.

OCM

lO

~

H

nW F o

6

K IIIIi

~

2

[8

I

:I

I

I

I

I

I

KiOll I

I

I

(c) IO

O--5CM

I

K~.IB

v K~011~m ~I
I

\

vv K ~ o 3 R~I K~OI I

I

I

I

I

i

%% TIME

Fig. 16. Summary of temporal changes in bacterial standing stocks, normalized to ~63 pm fraction, in the top 0.5 (A), 0.5-1.0 cm (B) depth intervals, and integrated over the upper 5 cm (C) at HEBBLE site stations showing significant elevation during K101 and K103 times of intermediate current velocities over periods of weak flow (K96, K126) or very strong flow (K106). Data from each station are equal to the mean + S.D. of nine separate subcore samples from each box core. Newman--Keuls multiple range test with unequal sample sizes and a 5% significance level rejects the hypothesis of equal bacterial numbers at each station at all five times: H0: ul :~ u2 #: u3 :~ u4 4: us. In (B) H0: ul = u2 = u3 :# u4 = us.

p a r t i c l e r e w o r k i n g (RHOADS e t al., 1985). T h i s c a n b e a c c o m p l i s h e d b y r a p i d l y b u r y i n g a s u r f a c e c o n t a c t with a n i n t e r v a l o f n e w l y d e p o s i t e d s e d i m e n t d u r i n g p e r i o d s o f high s e d i m e n t l o a d s a n d d e c e l e r a t i n g c u r r e n t s , o r t h e s e a b e d can b e r e s u s p e n d e d a n d r e d e p o s i t e d t o a d e p t h greater t h a n t h e b i o l o g i c a l r e w o r k i n g d e p t h ( g e n e r a l l y 3 - 1 0 c m o r greater) by a large storm event. Generally, only the largest storms can produce contacts t h a t a r e b u r i e d a n d p r e s e r v e d b e l o w t h e d e p t h o f m o s t i n t e n s e b i o t u r b a t i o n (i.e.

924

J.Y. ALLER A

. 5Cbi

36t0

--0 .

28]-

K103

~

20-

x--I

Z

Z

12-

I

K101

K96

0}

4 {B

I

t

L

I

t

t

I

I

.

EL t
0 . 5--9..

0Ci'.4

3{5-

-;r

28-

-'~ Kt03

~-20-

T

--~ K101 K96

\

I

Kt06

-i-

I

"T" ~T-

I

I

"~/43"~,z, " 3

I "

I

%

436

K~2S

I

I

I

%

436

\ Time Fig. 17. Summary of temporal changes in total sediment ATP in the top 0.5 (A) and 0.5-1.0 cm (B) depth intervals at HEBBLE site stations demonstrating significant elevation at times of intermediate current velocities. Each value is the mean + S.D. of nine separate subcore samples. Newman-Keuls test found Ho: u~ ~ u2 -~ u3 ~ u4 4= u5 at the 5% significance level.

- 1 0 cm). Assemblages of small infaunal pioneering species living in the top few cm of sediment are often not very effective in deep particle reworking (RHOADSet al., 1977), and therefore small-scale resuspension or depositional events can be recorded and preserved in high disturbance regimes (REINECKet al., 1967; RHOADSand BOYER, 1982). The mm-to-cm-thick laminations seen in numerous radiographs collected over a period of years from the HEBBLE site and elsewhere on the rise (YINGST and ALLER, 1982; unpublished data), reflect both depositional and erosional events on time scales of days, weeks and months. Spatial variability in the depth to the first major laminations (Zi) reflects differences in seabed microtopography and filling-in of natural topography by a

Table 5. Comparison o f macro- and meiofaunal (~>41 Ixm) standing stocks in sediments from Sta. V, 4817 m, H E B B L E site during July 1982 (K96) and April 1983 (K101). Data are the mean +- S.D. number o f individuals per 23 c m 2 of nine 23 cmz subcores from one 0.25 m 2 box core x + S.D. Taxa

July

Depth = 0-0.5 cm Nematoda Foraminifera Crustacea Copepod juveniles Ostracods Decapod juveniles Amphipods Polychaeta Bivalvia Kinorhyncha

443 118 14 18 1.8 1.8 111

Total

708

Depth = 0.5-1 cm Nematoda Foraminifera Crustacea Copepod juveniles Ostracods Polychaeta Sipunculida Bivalvia

April

+ 221 + 45 + 15 + 24 + 3.5 + 3.5 0 + 39 0.9 0

198

+ 116 3.6 12 + 14 0.9 5.4 + 5.7 0 0 6 + 9 6 + 12.5 0

+ 262

232

110 10 5 4

+ 63 + 9 + 6.5 + 6.6 0.9 1.8 + 3.5 0 0

Total

132

CaCO 3

i

IzI

I

132

+ 136

BREAK

5

I

117 + 125 5.3 + 7 3.5 + 3.5 0 0 4.4 + 8 1.8 + 3.5 0.9

64

(%)

3

I

+

+ 128

I

7

I

I

9

I

I

I

• 3

b

b

z2

b bb

Y O

5

I H 0_ W 7 o

b b []

[]

z 3 b

b

O

9

Fig. 18. Correlation of the depth of erosional contact with + 5 % breaks in % C a C O 3 from box cores collected on five cruises to the H E B B L E site. A general increase in % C a C O 3 with depth in these sediments is also demonstrated.

926

J.Y. ALLER

mobile sediment layer. The more prominent cross-laminations or contacts throughout the deposit have been shown to be correlated with +5% changes in %CaCO3 (Fig. 18), presumably because erosional events have caused the loss of fines by selective washing, leaving a layer or residuum of sand-sized particles composed almost entirely of calcareous foraminiferan tests.

Physical reworking, deposition, and erosion as the dominant particle reworking processes The preservation of physical and biological structures observed in X-radiographs, the sedimentologic data, and the radiochemical data of DEMASTERet al. (1985) indicate that both biogenic and physical processes are responsible for reworking of HEBBLE sediments. One interpretation of the data is that biogenic reworking usually dominates between - 5 and 15 cm depth, as demonstrated directly by preserved structures, while physical processes typically dominate the upper few centimeters ( - 0 - 5 or up to 10 cm) of these deposits. The counter position favors biological reworking as the dominant process controlling particle transport in surficial, as well as deeper, zones; thereby accounting for calculated particle mixing coefficients (DB) of up to --33 cm 2 y-l, based on radionuclide profiles in the upper 5-10 cm of these sediments (DEMASTER et al., 1985, 1987). The generally high abundance of near-surface animals an d presence of small burrows crisscrossing laminae in the top few cm is used to support this latter interpretation. Because 234Th is adsorbed onto particles from seawater and has a short half-life (tl/2 = 24 days), the distribution of this natural radionuclide can be an extremely sensitive indicator of short-term (100-day time scale) particle inputs, reworking and accumulation rates at the seafloor (ALLER and COCHRAN, 1976; ALLER and DEMASTER, 1984). The concentration gradient of excess 234Th can be modeled to obtain estimates of particle mobility. The distribution of the nuclide does not in itself allow differentiation of diffusive vs advective transport, as the mode of transport and boundary conditions must be assumed in any model calculations. Calculated mixing coefficients (Dn) are therefore indicative of penetration rate, but not necessarily the mode of transport. An equivalent "apparent sedimentation rate (0~)", is related to a calculated DB from a profile through: co -- ~X/-~--DBwhere ~ -- a decay constant. At the HEBBLE site, excess 234Th inventories increase as a function of the calculated particle mixing coefficient (Dn), illustrating trapping of newly resuspended particles during either biogenic reworking or rapid deposition (Fig. 19A). "Mixing coefficients" were also greatest during inferred depositional periods (K96) (thickly laminated zone) and lowest during inferred erosional periods (K106) (thinly laminated zone). Both the depth of penetration of excess 234Th and the excess 234Th inventories in sediments (DEMASTER et al., 1985, 1987; BREWSTER,1987) from all cruises are positively correlated with the maximum thickness of the surface laminated zone as seen in X-radiographs (Fig. 19B,C). The correlation of the 234Th inventory and penetration depth with physically produced fabrics strongly suggests the dominance of physical reworking, resulting in fairly rapid deposition (high burial rate, a high inventory and deep penetration). Erosion on the other hand results in a low inventory and shallow penetration depth of 234Th. These processes apparently take place on dimensions of several cm-thick layers and on time scales of weeks to months. Because of the short half-life of 234Th it is ideal for studying benthic and water column processes which take place on time-scales like those shown in Fig. 10. High particle mixing coefficients (DB), 2-33 cm 2 y-1 (Fig. 19A), and associated

Quantifying sediment disturbance by bottom currents

(A) 4~

927

[3 K~03

40

~.

35

~ ,~ g> g

~0

~

20

2s

El_

K96

Od 5

i

KI~5AK~O~',-

O' 10

20

30

l

40

i

~

k

50

60

70

l

90

80

Ob (cm2 yr -~) V e r t i c a l Mob~lity 234

(B) 0

Th Penetration Depth (cm]

0

2 ,

4 ,

4t

K126

6 ,

8 ,

~.0 ~.2 14 , , ,

234Th Excess Inventory (dpm cm-2)

'(C)" 'I6 ,

~.B 0 -

K103

5 ,

10 ,

15 ,

2 i , ! A KI06

KI06

4.

|

/

6' I ) O ' " -

20 i

25 ,

30 ,

35 ,

40 ,

45

KI03

IV" /

• ! ~

w

B K~26

E ~CZ

K96

10

•

K96 12

•

~4

~4

•

~5

~.6

~.8

18

~E

Fig. 19. (A) 234"naexcess inventories from three cruises to the HEBBLE site as a function of the calculated sediment mixing coefficient (vertical mobility) illustrating greater trappi_nlg"of resuspended particles and higher mixing rates during depositional periods (i.e. K96). (B) excess plotted against the maximum depth of the laminated layer. Correlation demonstrates deepest penetration during inferred rapid depositional periods (K96, K126). (C) 2a4Thxs inventories as a function of the maximum depth of the laminated layer in box cores collected on four cruises. Rapid deposition during periods of relatively slow currents correlates with highest 2a4Th,.~. The high degree of temporal variability at this site is apparent when inventories from Sta. XIII (in parentheses) at the four sampling times are compared. Spatial variability during one cruise can be assessed by comparing replicate subsamples from individual box cores taken at two stations (circled by _ _ _) during K96.

intensive biogenic reworking are consistent with the relatively high densities of benthic macrofauna (YINOST and ALLER, 1982; THISTLEet al., 1985; ALLER and ALLER, 1986a). These observations support a hypothesis of rapid biologic particle mixing as proposed by DEMASTER et al. (1985, 1987). However, if biological mixing is largely responsible for the observed particle mixing, it is highly unlikely that distinct millimeter-scale crosslaminations would be preserved. These laminae would be mixed and disrupted on vertical scales of about 1 cm within 1 month at measured effective diffusion rates. Additionally, surface 234Th values vary widely .across the 2 x 4 km HEBBLE site,

928

J . Y . ALLER

ranging from 3.2 to 18 dpm g-1. This variation may reflect localized differences in sediment supply, transport and variable resuspension rates due to physical processes as well as patchy distributions of infauna. The sum of these observations argues in favor of physical dominance of reworking in surface sediments in the H E B B L E area.

Conceptual model of biological community response to hydrodynamic conditions Bacterial standing stocks, along with macro- and meiofaunal abundances, are higher in sediments on the Nova Scotian Rise than at other deep-sea sites at comparable water depths (e.g. YINGSTand ALLER, 1982; THISTLEet al., 1985; ALLER and ALLER, 1986a). In the present case, a regular pattern of sediment bacterial abundance, microbial activity and possibly growth rates can be related to sedimentation processes and variations in the hydrodynamic regime, as recorded in time-averaged sedimentary features. Total sediment ATP concentrations, which reflect macro- and meiofaunal as well as microbial biomass (i.e. YINGS'r, 1978) and microbial physiological state/activity (i.e. KNOWLESand SMITH, 1970; DAVIS and WHrrE, 1980), show a significant increase between July 1982 (K96), a period of low current velocities, and April 1983, a time of intermediate current velocities (Fig. 3). This increase tracks greater bacterial numbers (Fig. 16), but lower macro- and meiofaunal abundances (Table 5). Moreover, an elevation in ATP/bacteria ratios occurs during times of intermediate current velocities (K101 and 103) in the top 00.5 and 0.5-1.0 cm depth intervals (Fig. 20). This phenomenon is consistent with a "mechanical stimulation" effect enhancing activity and presumably growth rates of those bacteria that were not removed by the stronger flow. The reduction in macro- and meiofaunal abundances in the topmost sediment layer during April 1983 (K101) is most likely the result of removal by currents. There is no indication of vertical migration into the sediment to avoid suspension into the overlying water because densities are unchanged below the top 0-0.5 cm (Table 5). Macro- and meiofauna entrained in bottom waters presumably settle out as current velocities decrease to ca < 5 cm s-1 along with settling resuspended sediment (HANNAN, 1984). Increased deposit feeding by these newly introduced populations could explain the decrease in bacterial numbers during periods of relative calm. At the other extreme, during periods of very strong currents (i.e. >25 cm s-l), surficial bacteria associated with particles, as well as macro- and meiofauna, are transported laterally, resulting in greatly reduced abundances of all size categories of organisms. These preliminary observations are summarized in a conceptual model relating current (shear stress)/sedimentation regime on the benthic community (Fig. 21) based on three kinetic regimes: depositional, transitional and erosional. Depositional regimes. These regimes, recorded by cm-thick layers of physically reworked sediment seen in X-radiographs (i.e. Fig. 7B), occur when recent current velocities are relatively low (<10 cm s-l). Vertical and horizontal burrow networks crisscross the top several cm of the deposit during these times, suggesting recent biogenic activity within a preceeding period of days to weeks. This fine-scale bioturbation does not entirely obliterate laminations. Standing stocks of bacteria and sediment ATP concentrations are intermediate between periods of moderate and very strong near-bottom currents. Labile organic material is presumably present at the sediment surface by lateral advection of particulate organic matter (POM) by benthic storms (Table 3). The time lag between benthic storms ( - every 21 days for storms with current velocities between 15 and 23 cm s-1) (Fig. 6) is approximately the same time scale, based on extrapolation of

Quantifying sediment disturbance by bottom currents

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shallow water rates, required for decomposition of the most reactive POM in sediments (SKOPINS~V, 1981; WESTRICH and BERNER, 1984). Thus depositional frequency appears to be sufficiently frequent to "fuel" the benthic community. A lateral/vertical source (as opposed to local resuspension) of fresh POM also takes place, as fresh undecomposed diatom debris from surface waters (ALLER and ALLER, 1986). This organic material settles out on the seabed during low velocity periods. Such conditions were observed in June and July of 1982 when relic burrows at the H E B B L E site were found to be filled with highly reactive, fresh diatom debris (ALLER and ALLER, 1986a). One would predict that, in the absence of the periodic and frequent influx of fresh organic matter, food resources would be depleted rapidly and metabolites would build up in sediments,

930

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Fig. 21. Conceptual model of the response of the bacterial community in western boundary regions such as the HEBBLE area, to temporal fluctuations in the hydrodynamic regime. Variations in current velocities (or bed shear stress) (A) correspond to depositional, transitional, or erosional periods (B), and are reflected in significant differences in bacterial standing stocks and microbial activity.

Quantifyingsedimentdisturbanceby bottomcurrents

931

leading to depression of microbial growth and activities (ALLER and YINGST, 1985; ALLER, 1987). Transitional regimes. These regimes mark times of intermediate levels of bottom current disturbance, when current velocities range from ~10 to 20 cm s-1. These transitional events typically occur on time scales of several weeks. During these times, surficial sediments are characterized by: (1) increased sand:silt/clay in the topmost centimeter (Fig. 13); (2) decreased POM in surface sediments (Fig. 14); (3) significantly decreased macro- and meiofaunal abundances in the topmost sediment layer (Table 5); (4) increased bacterial abundances in the top 1 to several centimeters (Fig. 16); and (5) increased ATP concentrations in these same sediment layers (Fig. 17). Mechanical disturbance by near-bottom currents therefore appears to counteract lowered POM availability enhancing microbial production and stimulating metabolic activities (e.g. GREENWOOD, 1968; ALLER, 1982). Bacterial standing stocks and sediment ATP concentrations, in fact, reach maximum levels in the top centimeter during these transitional periods. Current velocities are strong enough to erode silt/clay-sized particles and suspend meiofauna, many macrofauna and bacteria associated with fine particles (e.g. BELL and SHERMAN, 1980; SHERMANand COULL, 1980; PALMER and BRANDT, 1981). Surficial bacterial populations that remain apparently are stimulated by these transport events. Microbial metabolic rates in sediments that have been mechanically disturbed by vigorous stirring in the laboratory are significantly enhanced for periods of 1-2 weeks, compared with unstirred material (WESTRICH, 1983; ALLER, 1988). The reduction of inhibitory metabolite concentrations by resuspension in a moderate flow regime also stimulates microbial activities (ALLER and YINGST, 1985; ALLER, 1987). In spite of reduced organic C and N concentrations during periods of moderately strong flow and net erosion, concentrations on the Rise are still generally higher in the surficial sediments than in many deep-sea regions, and the C/N ratio (ranging from 6 to 10) suggests that the organic matter is still labile (YINGSTand ALLER, 1982; ALLERand ALLER, 1986a). Erosional regimes. These regimes are characterized by near-bottom currents reaching I>25 cm s-1 for several days (i.e. K106, Fig. 9). These events result in: (1) progressive loss of cross-laminations within the deposit by scouring; (2) a scour residium with elevated CaCO3 level in the top 1-2 cm; (3) an increase in sand relative to silt-clay-sized particles; (4) decreased organic matter content of the surface layer; (5) low bacterial abundances and ATP concentrations; and (6) presumably minimum net growth of the remaining biota. This scouring event exposes generally older refractory organic material and a coarse scour residium is left at the surface sediment. The overall consequence of the alternating erosional-depositional regime on the Scotian Rise is that the mean carrying capacity (density) of benthos is higher a priori than would be expected for this water depth in a lower energy deep-sea environment (THISTLEet al., 1985).

CONCLUSIONS Erosional and depositional events in this western boundary region are recorded in physical features of the seabed that can be assigned an operational magnitude (thickness of bedded sediment) and correlated with a driving force (current and bed shear stress). This latter correlation allows inference of the frequency and duration of benthic

932

J.Y. ALLER

disturbances using current records. Both erosional and depositional events cause perturbations which affect the benthos, but these disturbances are of different degree and kind. During erosional periods, sedimentary material, organic matter, as well as microorganisms, larvae and juveniles are transported away from the area. During transitional periods of intermediate current velocities, there is a vertical and horizontal influx of fresh organic material, diffusing metabolites are removed from the surface, and mechanical stimulation of the seabed by the currents increases microbial abundance and activities. Reduced grazing by lowered numbers of macro- and meiofauna also may contribute to increased standing stocks of bacteria. The shift from periods of high to low current magnitudes corresponds to periods of rapid sediment deposition (MCCAvE, 1983; GRANT et al., 1985). These events can also cause disturbance by burying organisms or by filling in burrows excavations. Nevertheless, during these depositional periods, both benthic macro- (i>297 Ixm) and meiofauna (I>41 lam) take advantage of increased food availability and reach peak abundances. The conceptual framework developed here provides a method for characterizing the current-driven disturbance in this western boundary region, a way to relate quantitatively (i.e. statistically) changes in the flow regime to both passive and active responses of the benthic community. Western boundary regions like the HEBBLE area are another example of a physically controlled deep water habitat (sensu SANDERS, 1977). Taking into account other areas, e.g. gravitational slumping and sliding, and hydrothermal vents (JUMARS and HESSLER, 1976; GRASSLE et al., 1985; GRASSLE, 1986), it now appears that the supposedly biologically controlled deep sea encompasses highly disturbed regions and patches, comparable in some ways to shallow water. Acknowledgements--I thank all those who assisted in collection and processing of box cores, particularly the crew of the W . H . O . I . R . V . Knott, R. Chandler and D. Thistle. B. Negele helped with sampling and Xradiography. Special thanks to G. Weathedy, T. Gross, S. Williams and the BUOY group, W.H.O.I. for current meter records. N. McCave's criticisms helped provoke my written integration of available data from many HEBBLE investigators. Discussions with R. Aller were invaluable and I appreciate his encouragement. D. Rhoads and R. Hessler provided valuable comments. Samples were obtained as part of the HEBBLE project (ONR contract no. N00014-80-C-0037 (J. Y. Aller, P.I.). Laboratory efforts were supported in part by NSF grant Rll-84-10278 (J. Y. Aller, P.I.). REFERENCES ALLEN S. E., H. M. GRINSHAW, J. A. PARKINSON and C. QUARMBY (1974) Chemical analysis of ecological materials. Blackwell Scientific, Oxford, U.K., 360 pp. ALLER R. C. (1982) The effect of macrobenthos on chemical properties of marine sediment and overlying water. In: Animal sediment relations, P. L. MCCALL and M. J. S. TEVESZ, editors, Plenum Press, New York, pp. 53-102. ALLER R. C. (1987) Stimulation by macrofauna of ammonia release in the bioturbated zone of marine sediments. LOS, 68, 1780. ALLER R. C. (1988) Benthic fauna and biogeochemical processes in marine sediments: The role of burrow structure. In: Nitrogen cycling in coastal marine environments, T. H. BLACKBURN and J. SORENSEN, editors, John Wiley, New York, pp. 301-338. ALLER R. C. and J. K. COCHRAN (1976) 234Th]238Udisequilibrium in near-shore sediment: particle reworking and diagenetic time scales. Earth and Planetary Science Letters, 29, 37-50. ALLER R. C. and D. J. DEMASTER (1984) Estimates of particle flux and reworking at the deep-sea floor using Z34Th/238Udisequilibrium. Earth and Planetary Science Letters, 67, 308--318. ALLER R. C. and J. Y. YINGST (1985) Effects of the marine deposit-feeders Heteromastus filiformis (polychaeta), Macoma balthica (bivaivia) and Tellina texana (bivalvia) on averaged sedimentary solute transport, reaction rates, and microbial distributions. Journal of Marine Research, 43, 615-645. ALLER J. Y. and R. C. ALLER (1986a) Evidence for localized enhancement of biological activity associated with tube and burrow structures in deep-sea sediment at the HEBBLE site, western North Atlantic. Deep-Sea Research, 33, 755-790.

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ALLER J. Y. and R. C. ALLER (1986b) General characteristics of benthic faunas on the Amazon inner continental shelf with comparisons to the East China Sea. Continental Shelf Research, 6, 291-310. ARMESTO J. J. and S. T. A. PICKETF (1985) Experiments on disturbance in old-field plant communities: impact on species richness and abundance. Ecology, 66, 230-240. BELL S. S. and K. S. SHERMAN (1980) Tidal resuspension as a mechanism for meiofauna dispersal. Marine Ecology Progress Series, 3, 245-249. BISCAYE P. E., W. D. GARDNER, R. J. ZANEVELD,H. S. PAK and B. TUCHOLKE (1980) Nephels! Have we got nephels! LOS, 61, 1014. BREWSTER D. C. (1985) A radiochemical investigation of a dynamic deep-sea environment: The HEBBLE site. Masters Thesis, NC State University, Raleigh, North Carolina, 46 pp. BULLEID N. C. (1978) An improved method for the extraction of adenosine triphosphate from marine sediment and seawater. Limnology and Oceanography, 23, 174-178. DAVIS W. M. and D. C. WHITE (1980) Fluorometric determination of adenosine nucleotide derivatives as measures of microfouling, detrital, and sedimentary microbial biomass and physiological status. Applied and Environmental Microbiology, 40, 539-548. DEMASTER D. J., B. A. McKEE, C. A. NrrrROUER, D. C. BREWSTER and P. E. BISCAYE (1985) Rates of sediment reworking at the HEBBLE site based on measurements of Th-234, Cs-137, and Pb-210. Marine Geology, 66, 133-148. DEMASTER D. J., D. C. BREWSTER, C. A. NITrROUER and B. A. MCKEE (1987) Particle-scavenging and particle-reworking on the Nova Scotian Continental Rise: a four-year summary of radiochemical data from the HEBBLE site. LOS, 68, 1747. FOLK R. L. (1974) Petrology of sedimentary rocks. Hemphill Publishing Co., Austin, Texas, 182 pp. GARDNER W. D., P. E. BISCAYE, J. R. V. ZANEVELD and M. S. RICHARDSON (1985) Calibration and comparison of the LOGO Nephelometer and the OSU transmissometer on the Nova Scotia Rise. Marine Geology, 66, 323-344. GAUDETI'E H. E., W. R. FLIGHT, L. TONER and D. W. FOLGER (1974) An inexpensive titration method for the determination of organic carbon in recent sediments. Journal of Sedimentary Petrology, 44, 249-253. GRANT W. D., A. J. WILLIAMS, III and T. F. GROSS (1985) A description of the bottom boundary layer at the HEBBLE site; low-frequency forcing, bottom stress and temperature structure. Marine Geology, 66, 217-241. GRASSLE J. F. (1986) The ecology of deep-sea hydrothermal vent communities. Advances in Marine Biology, 23, 301-362. GRASSLE J. F., L. S. BROWN-LEGER, L. MORSE-PORTEOUS, R. PETRECCA and I. WILLIAMS (1985) Deep sea fauna of sediments in the vicinity of hydothermal vents. Bulletin of the Biological Society of Washington, 6, 443--452. GREENWOOD D. J. (1968) Measurement of microbial metabolism in soil. In: The ecology of soil bacteria, T. R. G. GRAY and D. PARKINSON, editors, University of Toronto Press, pp. 138-157. HArCNAN C. A. (1984) Planktonic larvae may act like passive particles in turbulent near-bottom flows. Limnology and Oceanography, 29, 1108-1116. HARVEY H. R., M. D. RICHARDSON and J. S. PATRON (1984) Lipid composition and vertical distribution of bacteria in aerobic sediments of the Venezuela Basin. Deep-Sea Research, 31,403-413. HESSLER R. R. and P. A. JUMARS (1974) Abyssal community analysis from replicate box cores in the central North Pacific. Deep-Sea Research, 21, 185-209. HOBBIE J. E., R. J. DALLY and S. JASPER (1977) The use of nuclepore filters for counting bacteria by fluorescence microscopy. Applied and Environmental Microbiology, 33, 1225-1228. JUMARS P. A. and R. R. HESSLER (1976) Hadal community structure--Implications from the Aleutian Trench. Journal of Marine Research, 34, 547-560. KNOWLES C. J. and L. SMITH (1970) Measurement of ATP levels of intact Azotobacter vinelandii under different conditions. Biochimica et Biophysica Acta, 197, 152-160. MCCAVE I. N. (1983) Particulate size spectras, behavior and origin of nepheloid layers over the Nova Scotian continental rise. Journal of Geophysical Research, 88, 7647-7666. MCCAVE I. N. (1985) Sedimentology and stratigraphy of box cores from the HEBBLE site on the Nova Scotian Continental Rise. Marine Geology, 66, 59-89. MtmK W., F. SNODGRASSand M. WIMBUSH(1970) Tides off-shore transition from California coastal to deepsea waters. Geophysical Fluid Dynamics, 1, 161-235. PALMER M. A. and R. R. BRAtCDT (1981) Tidal variation in sediment densities of marine benthic copepods. Marine Ecology Progress Series, 4, 207-212. PELT R. K., D. C. GLENN-LEWINand J. W. WOLFF (1983) Predictions of man's impact on vegetation. In: Man's impact on vegetation, W. NOLZNER, M. J. A. WAGER and R. G. HURD, editors, Junk, The Hague, pp. 41-53. PICKETR S. T. A. and P. S. WHITE (1985) The ecology of nalural disturbance and patch dynamics, S. T. A. PICKETR and P. S. WHITE, editors, Academic Press, New York, 472 pp.

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REINECK H. E., W. F. GtrrMnn and G. HERTWEEK (1967) Das Schlickgebiet sudlick. Helgoland als Beispiel regenter Schelfablagerungen Senckenbergiana Lethaea, pp. 219-275. RHOADS O. C. and L. F. BOYER (1982) The effects of marine benthos on physical properties of sediments: a successional perspective. In: Animal-sediment relations, M. J. S. TEVESZ and P. L. McCALL, editors, Plenum Press, New York, pp. 31-43. RHOADS D. C., R. C. ALLER and M. B. GOLDHABER (1977) The influence of colonizing benthos on physical properties and chemical diagenesis of the estuarine seafloor. In: Ecology of marine benthos, B. C. COULL, editor, University of South Carolina Press, Columbia, pp. 113-138. RHOADS D. C., P. L. MCCALL and J. Y. YINGST (1978) Disturbance and production on the ecology of the estuarine seafloor. American Scientist, 66, 577-586. RItOADS D. C., D. F. BOESCH, T. ZHICAN, X. FENGSHAN, H. LIGIANG and K. J. NILSEN (1985) Macrobenthos and sedimentary facies on the Changjiang Delta platform and adjacent continental shelf, East China Sea. Continental Shelf Research, 4, 189-213. RICHARDSON M. J., M. WIMBUSH and L. MAYER (1981) Exceptionally strong near-bottom flows on the continental rise of Nova Scotia. Science, 213, 887-888. RowE G. T. and J. W. DEMING (1985) The role of bacteria in the turnover of organic carbon in deep-sea sediments. Journal of Marine Research, 43, 925-950. RUSSELL D. E. (1987) Paedamphaete acutiseries, a new genus and species of Ampharetidae (Polychaeta) from the North Atlantic (HEBBLE) area exhibiting progenesis and broad intraspecific variation. Biological Society of Washington Bulletin, 7, 140-151. SANDERS H. L. (1977) Evolutionary ecology and the deep-sea benthos. In: Changing scenes in natural sciences, C. E. GOULDEN, editor, Academy of Natural Sciences, Philadelphia, Pennsylvania, pp. 223-243. SANTOS S. L. and J. L. SIMON (1980) Marine soft-bottom community establishment following annual defaunation. Larval or adult recruitment? Marine Ecology Progress Series, 2, 235-241. SCmNK J. C., J. H. STOCKwELL and R. A. ELLIS (1979) An improved device for gasometric determination of carbonate in sediment. Journal of Sedimentary Petrology, 49, 651-653. SHERMAN K. M. and B. C. COULL (1980) The response of meiofauna to sediment disturbance. Journal of Experimental Marine Biology and Ecology, 46, 59-71. S~OPINSTEV B. A. (1981) Decomposition of organic matter of plankton humification and hydrolysis. In: Marine

organic chemistry: evolution, composition, interactions and chemistry of organic matter in seawater, E. K. DURSEMA and R. DAWSON, editors, Elsevier, New York, pp. 125-177. SOKAL R. R. and P. J. ROHLF (1969) Biometry, W. H. Freeman and Co., San Francisco, CA, 776 pp. SWIFT S. A., C. D. HOLLIS'rER and R. S. CHANDLER (1985) Close-up stereo photographs of abyssal bedforms on the Nova Scotia Continental Rise. Marine Geology, 66, 303-322. TENORE K. R., C. F. CHAMBERLAIN,W. M. DUNSTAN, R. B. HANSON, B. SHERR and J. H. TIETJEN (1978) Possible effects of Gulf Stream intrusions and control runoff on the benthos of the continental shelf of the Georgia Bight. In: Estuarine interactions, M. L. WILEY, editor, Academic Press, New York, 577-598. THISTLE D. (1983) The stability-time hypothesis as a predictor of diversity in deep-sea soft-bottom communities: a test. Deep-Sea Research, 30, 267-277. THISTLE D., J. Y. YINOST and K. FAUCHALD (1985) A deep-sea benthic community exposed to strong nearbottom currents on the Scotian Rise (Western Atlantic). Marine Geology, 66, 91-112. WATSON S. W., T. J. NOVISTSKY, H. L. QUINBY and F. W. VALOIS (1977) Determination of bacterial number and biomass in the marine environment. Applied and Environmental Microbiology, 33, 940-946. WEATHERLY G. L. and E. A. KELLEY, Jr (1983) "Too cold" bottom layers at the base of the Scotian Rise. Journal of Marine Research, 40, 985-1012. WEATHERLY G. L. and E. A. KELLEY, Jr (1985) Storms and flow reversals at the HEBBLE site. Marine Geology, 66, 205-218. WESTRICH J. E. (1983) The consequences and controls of bacterial sulfate reduction in marine sediments. Ph.D. Thesis, Yale University, 530 pp. WESTRICH J. E. and R. A. BERNER (1984) The role of sedimentary organic matter in bacterial sulfate reduction: The G model tested. Limnology and Oceanography, 29, 236--249. WHITLATCH R. B. (1977) Seasonal changes in the community structure of the macrobenthos inhabiting the intertidal sand and mud flats of Barnstable Harbor, Massachusetts. Biological Bulletin, 152, 275-294. YINGST J. Y. (1978) Patterns of micro- and meiofaunal abundance in marine sediments measured with the adenosine triphosphate assay. Marine Biology, 47, 41-54. YINGST J. Y. and R. C. ALLER (1982) Biological activity and associated sedimentary structures in HEBBLEarea deposits, western North Atlantic. Marine Geology, 48, M7-M15. YINGST J. Y. and D. C. RHOADS (1985) Benthic community structure in the vicinity of the Texas Flower Garden Banks, Gulf of Mexico. Estuarine, Coastal and Shelf Science, 20, 569-592. ZAJAC R. N. and R. B. WHITLATCH (1982) Responses of estuarine infauna to disturbance. I. Spatial and temporal variation of initial recolonization. Marine Ecology Progress Series, 10, 1-14.